1. Field of the Invention
This invention relates generally to the reproduction of sound in multi-channel systems generically known as “surround-sound” systems and more specifically to the application of psychoacoustic principles in the design of a loudspeaker system for reproducing a surround sound experience from loudspeakers located only in front of the listener.
2. Background Art
One problem which is common to all methods of producing phantom rear located sound images from a sound source or sources located in front of the listener is that the phantom source illusion tends to collapse as the listener turns or moves the listener's head even slightly. This problem arises from fundamental differences in the way sound at the listener's two ears changes as the head turns in relation to the location of a sound source in front of the listener as compared to a rear located sound source. For example, for a front located sound source not on the median plane (i.e., the vertical plane equidistant from the listener's two ears.), the interaural time delay ITD for sound arriving at the listener's ear nearest the sound source relative to the arrival time of the sound at the ear farthest from the sound source will decrease as the listener turns toward the sound source. Similarly, the interaural level difference ILD between the listener's two ears will also decrease as the listener turns toward the sound source. Both the ITD and ILD will be zero when the listener faces directly toward the sound source. In comparison, for a sound source located behind the listener, the ITD will increase as the listener turns towards the sound source and will reach a maximum when the listener has turned such that the sound source is located directly to one side, 90 degrees from the median plane. In general, the ILD will also increase as the listener turns towards a rear located sound source and will reach a maximum when the sound source is located directly to one side. However, as is well known, the behavior of the ILD at individual frequencies is complex and may not follow this general rule.
A second problem arises specifically in systems which use variations of a difference signal to create phantom rear located sound images such as is described in parent application Ser. No. 10/692,692. Difference signals are formed by subtracting one audio signal from a second audio signal. Referring to FIG. 1 herein, which corresponds to FIG. 2a of parent application Ser. No. 10/692,692, a device is shown for reproducing surround sound from front located speakers. In this device, left and right sub-speakers LSS and RSS receive difference signals (LS′−RS′) and (RS′−LS′), respectively. These difference signals are developed from the left and right surround signals LS and RS for the purpose of creating phantom rear sound images as described in more detail the parent application. However, whenever the two components of the difference signal are the same, the components substantially cancel each other and the difference signal becomes zero. In such a device, when the difference signal drops close to zero the device becomes much less effective in creating phantom rear located sound images. Unfortunately, this situation may occur quite often. For example, in surround systems using Dolby® Pro-Logic® decoding, both rear channels carry the same signal. Many two channel to five channel music surround schemes also use monaural rear channel signals. The situation also occurs in discrete five channel systems when the rear image is intended to be located directly behind the listener. This last situation is, of course, well known as one of the most difficult phantom images to produce.
An additional problem is encountered in systems using passive methods to develop a difference signal by modifying and combining the speaker level output of two or more amplifier channels. Such a system is disclosed in U.S. Pat. Nos. 4,683,505 and 4,759,066 to Polk, et al. As is disclosed in these patents, amplifier channels which do not share a common ground may be damaged if DC current flows are permitted from one channel to the other. However, the methods proposed for isolation of the channels involve costly transformers which may also degrade performance.